The present invention relates to synchronous switching voltage regulator circuits. More particularly, the present invention relates to polyphase synchronous switching regulator circuits that provide lower peak input current, lower root-mean-square (RMS) input ripple current, lower RMS output ripple current, higher slew-rate capability and less stringent input and output filter requirements compared to comparable single-phase synchronous switching regulator circuits.
Switching regulators provide a predetermined and substantially constant output voltage from a source voltage that may be poorly-specified or fluctuating, or that may be at an inappropriate amplitude for the load. Such regulators typically employ a switch that includes one or more switching elements coupled in series or in parallel with the load. The switching elements may be, for example, power metal-oxide semiconductor field-effect transistor (MOSFET) switches. Control circuitry regulates the current supplied to the load by varying the ON-OFF times of the switching elements (i.e., the regulator's duty cycle, which is the percentage of time that a switch is ON during a cycle of operation). Inductive energy storage elements typically are used to convert the switched current pulses into a steady flow of load current.
Synchronous switching regulators include a main switching element and a synchronous switching element that typically are driven by non-overlapping clock pulses to supply current at a regulated voltage to a load. Synchronous switching regulators that use power MOSFET switches frequently are used in portable battery-powered electronic products and thermally-sensitive products. These regulators convert the typically fluctuating input voltage to a regulated output voltage. Such regulators provide high operating efficiency and thus long battery life with little heat generation.
The duty cycle of a synchronous switching regulator typically is controlled by monitoring the regulator's output voltage. In particular, the control circuitry generates a feedback signal V.sub.e that is proportional to the difference between the regulator's output voltage and a reference voltage. V.sub.e may be used to provide either "voltage-mode" or "current-mode" regulation. In voltage-mode regulation, V.sub.e and a periodic sawtooth waveform V.sub.s are provided as inputs to a comparator, the output of which controls the duty cycle of the power MOSFET switches. For example, if V.sub.e is connected to the comparator's non-inverting input, V.sub.s is connected to the comparator's inverting input, and at the start of each clock cycle V.sub.s is below V.sub.e, then the comparator's output is HIGH, the main switch is ON and the synchronous switch is OFF. When V.sub.s exceeds V.sub.e, the comparator's output changes state to LOW, which turns OFF the main switch and turns ON the synchronous switch.
In current-mode regulation, a voltage V.sub.i is generated that is proportional to the current in the output inductor, and V.sub.i and V.sub.e are provided as inputs to a comparator, the output of which controls the duty cycle of the power MOSFET switches. For example, if V.sub.i is connected to the comparator's non-inverting input, V.sub.e is connected to the comparator's inverting input, and if at the start of each clock cycle V.sub.i is below V.sub.e, then the comparator's output is LOW, the main switch is ON and the synchronous switch is OFF. When V.sub.i exceeds V.sub.e, the comparator's output changes state to HIGH, which turns OFF the main switch and turns ON the synchronous switch. In this regard, the main switch turns OFF when the output current reaches its desired value.
Prior art synchronous switching regulators typically include a single power stage controlled by a single-phase clock. Such single-phase synchronous switching regulators are inefficient, however, for applications that require low voltage (e.g., &lt;2 volts) at high current (e.g., greater than 35 amps), such as power supplies for advanced microprocessors. In particular, because the input RMS ripple current in a switching buck regulator is approximately one-half the output current, high output current buck regulators require large input capacitors to provide sufficient input ripple filtering. Typically, this requires connecting many capacitors in parallel, which is costly and consumes valuable circuit board space.
In addition, for many switching regulator topologies, the input current has sharp discontinuities as a result of the switching action of the main switch. Because the regulator's peak input current is approximately equal to the circuit's output current, high current designs require substantial filtering to limit conducted electromagnetic interference (EMI). Further, for applications that require output currents greater than approximately 15 amps, the necessary output filter inductors are too large to be implemented using surface mount technology. Additionally, large output capacitors typically are required to minimize output ripple current.
Moreover, many state-of-the-art microprocessors have dynamic loading characteristics that frequently change from nearly zero load to full load within a very short time span, often tens to hundreds of nanoseconds. Power supplies for such microprocessors must have very high current slew rate capability to meet such dynamic load requirements and minimize the decoupling capacitance required at the load. High slew rate capability requires that the inductor value should be minimized. Because the output ripple current is inversely proportional to inductor value, however, prior art synchronous switching regulator circuits typically achieve high slew rate only at the expense of large output ripple current.
In view of the foregoing, it would be desirable to provide efficient, low voltage, high current, synchronous switching regulator circuits that have low input and output ripple currents.
It would further be desirable to provide low voltage, high current synchronous switching regulator circuits that limit conducted EMI.
It additionally would be desirable to provide low voltage, high current synchronous switching regulator circuits that may be implemented using surface-mount technology.
It also would be desirable to provide low voltage, high current synchronous switching regulator circuits that have high current slew rate capability and low output ripple current.